This invention relates to measurement of the rheological properties of viscoelastic materials, and to an improved method and apparatus for such measurement. The invention is particularly applicable to the measurement of the rheological properties of vulcanizable elastomers as they are in the process of being vulcanized.
Prior art devices for measuring the rheological properties of viscoelastic materials include the device disclosed in U.S. Pat. No. 3,681,980. This device encloses a sample of material to be tested in a chamber under pressure, and subjects the sample to shearing forces by means of an oscillating rotor located in the chamber and surrounded by the sample. The torque generated in shearing the sample is displayed as a continuous curve, relating torque to time. By means of a heated chamber, the behavior of the sample is monitored as it is subjected to heat over a span of time. In the case of vulcanizable elastomers, a "cure curve" is thus obtained which can be valuable in predicting the behavior of the elastomer during processing and vulcanization.
A characteristic of such systems employing a rotor is that the rotor is not heated, hence accurate temperature control of the sample is difficult. Additionally, when the sample is in place the sample surrounds the rotor and thus it must be initially loaded in two segments. A sample which vulcanizes during testing must then be removed from around the rotor necessitating a time-consuming manual operation.
Efforts to design a "rotorless" device for measuring rheological behavior of elastomers have met with a number of difficulties. It has been found that unless the system is sealed the sample becomes porous, and test results are nonreproducible. Sealed systems often result in excessive friction in the seal area, thus contributing a substantial, variable component of the overall torque, with consequent erratic test results. Since the torque generated by the known rotorless devices is usually less than that of the rotor systems, the change for error can be further aggravated.
Edge effects are another problem with known rotorless die systems, wherein the outer edges of the sample, which are subjected to the highest shear forces, tend to break away from the die surfaces, causing sample slippage and giving erroneous results. If the angle of oscillation is reduced to avoid the edge-effect problem, torque values are correspondingly reduced and accuracy is further impaired. Any factor which lowers the ratio of actual shear torque to extraneous torque (similar to a "signal-to-noise" ratio) has a derogatory effect on the accuracy of the system.
Thus, a need exists for a system of measuring the rheological properties of viscoelastic materials which eliminates the rotor, yet gives accurate, reproducible test results.